The hybrid-type construction machine ordinarily drives a hydraulic pump by the output of an engine (an internal combustion engine) and performs operations with generated hydraulic pressure. The engine is driven efficiently by assisting the engine with an electric motor. The electric motor is driven by electric power from a battery. The battery is chargeable and dischargeable and supplies electric power to the electric motor.
Meanwhile, if the engine is not assisted, electric power generated by the engine and regenerative electric power caused by the hydraulic load is electrically charged. With this, the electric motor can be assisted by the battery which is maintained by charging to be charged to some extent.
As described, the hybrid-type construction machine can assist the engine by the electric motor. Therefore, the maximum output required by the engine can be decreased. Thus, a small engine may be used. When an output larger than the maximum output of the engine is required, the electric motor may be used to assist the demand. The hybrid-type construction machine uses a variable displacement hydraulic pump so that hydraulic pressure is generated in response to a requirement of the hydraulic load (see Patent Document 1).